An Assessment of Grazing Effects on Soil Carbon Stocks in Grasslands

PIÑEIRO, G.; PARUELO, J.M.; OESTERHELD, M.; JOBBAGY, E.

Rangeland Ecology and Management (ex JRM)

SOC RANGE MANAGEMENT

Año: 2009 vol. 63

1550-7424

Grazing modifies the structure and function of ecosystems, affecting soil organic carbon (SOC) storage. Although grazing effects< on some ecosystem attributes have been thoroughly reviewed, current literature on grazing effects on SOC needs to besynthesized. Our objective was to synthesize the effects of grazing on SOC stocks in grasslands, establishing the majormechanistic pathways involved. Additionally, and because of its importance for C biogeochemistry, we discuss the controls ofsoil organic nitrogen stocks. We reviewed articles analyzing grazing effects on soil organic matter (SOM) stocks by comparinggrazed vs. ungrazed sites, including 67 paired comparisons. SOC increased, decreased, or remained unchanged undercontrasting grazing conditions across temperature and precipitation gradients, which suggests that grazing influences the factorsthat control SOC accumulation in a complex way. However, our review also revealed some general patterns such as 1) rootcontents (a primary control of SOC formation) were higher in grazed than in their ungrazed counterparts at the driest andwettest sites, but were lower at sites with intermediate precipitation (,400 mm to 850 mm); 2) SOM C:N ratios frequentlyincreased under grazing conditions, which suggests potential N limitations for SOM formation under grazing; and 3) bulkdensity either increased or did not change in grazed sites. Nearly all sites located in the intermediate precipitation range showeddecreases or no changes in SOC. We grouped previously proposed mechanisms of grazing control over SOC into three majorpathways that can operate simultaneously: 1) changes in net primary production (NPP pathway), 2) changes in nitrogen stocks(nitrogen pathway), and 3) changes in organic matter decomposition (decomposition pathway). The relative importance of thethree pathways may generate variable responses of SOC to grazing. Our conceptual model suggests that rangeland productivityand soil carbon sequestration can be simultaneously increased by management practices aimed at increasing N retention at thelandscape level.